An elevator installation usually comprises an elevator cage and a counterweight, which are moved in opposite directions in an elevator shaft. The elevator cage and the counterweight in this regard run along guide rails and are supported by at least one support means, which is guided over a driving drive pulley. The support means usually consists of one or more sheathed or unsheathed steel cables, one or more synthetic fiber cables, one or more flat or profiled belts (for example wedge-ribbed belts) or a parallel extending composite of the respective mentioned constructions, in which each individual support means can be guided over a respective individual drive pulley or a common drive shaft.
A very high traction on the driving drive pulley/drive shaft can be realized by modern support means, for example, when use is made of support means which are sheathed by synthetic material and which have a significantly higher coefficient of friction by comparison with classic steel cables.
Due to the high level of traction it is possible, for example, to continue raising the elevator cage although the counterweight could be blocked in its downward movement by unexpected jamming in the elevator shaft or unexpected seating on the shaft floor buffers. The same problem can arise with the counterweight if the elevator cage should sit on the shaft floor buffers. This lifting up of a load—be it the elevator cage or the counterweight—at one side of the drive pulley without the intended counter-load running conjunctively to freely drop at the other side of the drive pulley is undesired and can lead to risk-laden states, for example, dropping down of the elevator cage or the counterweight or tripping of maintenance personnel in the shaft head.
In correspondence with various regulatory standards and due to safety considerations use is therefore made of numerous so-termed slack-cable switches for recognition of corresponding risk situations in the elevator installation. Different risk situations can thus be recognized depending on the arrangement and construction of these switches.
Consequently, monitoring devices for detection of an unloaded, slack support means have been developed. They can be based, as disclosed in, for example, European application EP-A1-1 953 108, on a spring-reinforced mounting of the entire drive and a deflecting unit with at least two further rollers for the support means. This approach can be very costly.
International application WO-A1-2007/144456 discloses a direct, spring-reinforced version of the support means. WO-A1-2007/144456 discloses elevator equipment with an elevator cage, a counterweight and a 2:1 support means guidance, wherein an individual switch for detection of support means slackness is provided at each support means end. A relaxation, which arises at the fastening point of the support means due to load relief thereof, of a spring triggers the switch, which switches off the drive.
The disadvantages of these two prior art solutions can include on the one hand the constructional complication and on the other hand the costs.